Rack and Pinion axial aligning mechanism. Adjustment of this aligns the microscope body along the transmitted light optic axis. Note the hinged objective lens on the left side of the microscope support arm.
Axial motion affected by the adjustment R&P.
The Adams microscope had a fixed position microscope body and a movable stage. This rack and pinion focus mechanism moved the specimen relative to the objective lens.
The thumbscrew on the condenser lens mount (below the stage) is a jam screw to hold the lens in position.